Streams and ocean currents, such as tidal stream flows, provide a predictable and reliable source of energy that can be used for generating electrical energy. Stationary, or fixed, power plant systems are known which are submerged and secured in relation to the stream or flow, wherein a turbine is used to generate electrical energy from the flow velocity of the stream. A drawback with stationary stream-driven power plant systems, however, is that the amount of generated electrical energy from a single turbine of a certain size is low, which may be compensated by increasing the number of turbines, or increasing the effective area of the turbines. Those solutions, however, lead to cumbersome and expensive manufacturing, handling and operation of the fixed stream-driven power plant systems. Turbines may also be designed for installation in specific locations having high local flow speeds. This also leads to more complex and costly installation and handling. Moreover, access to such high flow speed locations is relatively limited.
In order to improve the efficiency of the electrical energy generation from tidal stream flows and ocean currents, it is known to provide a submersible power plant system comprising a stream-driven vehicle, as described in e.g. EP 1816345 by the applicant and fully incorporated herein by reference. The stream-driven vehicle typically comprises a wing which is designed to increase the speed of the vehicle by utilizing the stream flow and the resulting hydrodynamic forces acting on the wing. In more detail, the increased speed of the vehicle is achieved by counteracting the stream flow and hydrodynamic forces acting on the vehicle by securing the vehicle to a support structure, typically located at the seabed, by means of a wire member, wherein the vehicle is arranged to follow a certain trajectory which is limited by the length, or range, of the wire. The vehicle is further provided with a turbine coupled to a generator for generating electrical energy while the vehicle moves through the water, wherein the speed of the vehicle influences and contributes to the relative flow velocity at the turbine. The speed of the vehicle allows for that the relative flow velocity at the turbine may be considerably increased in relation to the absolute stream flow speed. Hence, the vehicle of the stream-driven submerged power plant does typically not require for use of a gear box, since the turbine is subjected to a high relative water speed which generates a sufficient, or more efficient, rotational speed for the generator.
One drawback associated with current solutions of power plant systems comprising a stream-driven vehicle is that the drag force acting on the power plant system reduces the efficiency of the electrical energy generation. An additional consideration of stream-driven submersible power plant systems is that they are mounted and arranged to operate in a submerged location, being difficult to access. Hence, handling and servicing of those systems is cumbersome. Accordingly, currently available, or known, stream-driven submersible power plant systems, and parts thereof, limit the maximal capability of efficiently generating electrical energy. Also, there is a desire to further develop the design and operation of such systems in order to facilitate for easier handling and for more efficient mounting, and to provide improved operation and more durable constructions.